For a few months now, Democratic U.S. Sen. Tammy Baldwin has been hitting Republican Leah Vukmir on the issue of insurance coverage for people with pre-existing medical conditions.

On Friday, the Baldwin campaign unleashed a new ad that triggered a quick and unusual response from Vukmir, who is trying to topple the first-term senator in the Nov. 6 midterm election.

In the ad, Shannon Thielman, a nurse and breast cancer survivor from Wausau, said that "it's upsetting that Leah Vukmir supports letting insurance companies go back to denying coverage to people with pre-existing conditions."

"As a nurse and a cancer survivor I just cannot support Leah Vukmir," Thielman said.

Vukmir, a nurse and state senator from Brookfield, responded with a letter to Thielman posted on Twitter. Thielman had taken over Baldwin's Twitter account.

Vukmir saluted Thielman for her "commitment as a nurse to serving patients and helping save lives."

Vukmir continued: "Sadly, Senator Baldwin is lying to you and all Wisconsinites when she says I want to harm people with pre-existing conditions. As a nurse like you, I’ve made it my life’s work to help patients and save lives. I’m sure we can agree that nurses want to help their patients and ensure they have access to the best healthcare possible."

"That’s why it’s incredibly misleading — and frankly false — that Senator Baldwin says 2.4 million Wisconsinites would lose their health insurance if we repeal Obamacare and let the states take care of healthcare," Vukmir wrote. "Is she really saying 40% of Wisconsinites wouldn’t have insurance anymore? Because we know that isn't true."

The Baldwin ad points out that 2.4 million Wisconsinites have pre-existing medical conditions but there is no mention that all would lose coverage if the Affordable Care Act is repealed.